I haven't figured out what in beersmith tweaks that pre-boil gravity number. Mine is consistantly off by about 10 points (I always have a higher number than BS says I should), but end up within 1-3+/- of my original gravity target and I don't run off 15.57 gallons in my 15.5 gallon keggle as it tells me to (so it is not at all concerning).

I figure for me it is a combination of the equipment settings in BeerSmith, (it doesn't have a keggle setting), an estimation of what my deadspace in the mash tun is (I never checked), and that for whatever reason, my vigorous boil does not evaporate at the same rate that BS thinks it should....(I am at altitude, but I would figure my boil off rate would be higher, not lower, but boil temp is lower here, too).

At any rate, I feel like hitting your expected pre-boil gravity is an indication that you have done everything properly and all systems are go!

I take a preboil gravity reading for every batch since I know exactly what my preboil volume is. When I take my post boil gravity I can calculate the exact post boil volume thus getting a more accurate efficiency. My buckets are only calibrated in 1/2 gallon increments so I like to know my post boil volume more accurately...